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Guide For Approximate Zone A Areas Developing- BFEs community's Flood Insurance study. The greater the level of detail on the topographic map, the more accurate the BFE determination. If the community does not have a Flood Insurance study, an existing topographic survey should, at a minimum, be as detailed as the U.s. Geological Survey quadrangle map for the area. Regardless of the level of detail of the existing- topographic map used, it is suggested that the g-eometry of the actual stream channel be obtained by a site visit if the cross sections are to be used for hydraulic analyses. Datum Requirements for Field surveys If a greater level of detail is desired than is available from existing topographic mapping, then a field survey should be performed. If it is necessary to establish a BFE for insurance purposes or to meet the requirements of 6D.3 of the NFIP Regulations, the survey must be referenced to the same datum that is used to produce the FIRM, which is usually the National Geodetic vertical Datum of 1929 or the North American vertical Datum of 1988. Reference marks giving elevations to this datum are given in the published Flood Insurance Studies. If the reference marks cannot be located in the field, or are simply too far away, additional reference mark information may be obtained from the state's U.s. Geological Surveyor Transportation office. Local surveyors are generally familiar with nearby reference marks. In approximate Zone A areas, if it is not economically feasible to reference survey information to a known reference mark, an assumed datum may be used, provided that the BFE, structure, and lot elevations are referenced to the same assumed datum; however, data developed using such an assumed datum may not be sufficient to revise a FIRM. All surveys must be certified by a registered professional engineer or land surveyor. If the sole purpose of determining relative flood heights is to meet the requirements set forth in section 60.3(a) of the NFIP regulations, any assumed datum may be used. In this instance, a depth of flooding can be established at a particular location without having to reference it to a datum (i.e., National Geodetic Vertical Datum). ~owever, in order for an insurable structure to be eligible for a lower insurance rate based on the BFE, the survey may need to be referenced to the same datum that was used for the FIRM (i.e., National Geodetic Vertical Datum or North American vertical Datum) . ~ V-12